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JEM-EUSO Atmospheric Monitoring System

JEM-EUSO will use the Earth’s atmosphere as a high-energy particle detector. The raw data will be information about the temporal and spatial variation of the intensity of UV light produced by EAS propagating through the atmosphere. These data will be used to derive the parameters of the primary UHECR particles such as the arrival direction, the energy and also the type of the UHECR particle (proton or heavier atomic nuclei, photons and/or neutrinos).

At the same time, the intensity of the UV light coming to JEM-EUSO from different parts of the shower depends not only on the properties of the UHECR particles, but also on the properties of the atmosphere, in particular on its transmittance and scattering characteristics in the UV band. These characteristics are strongly variable as a function of meteorological conditions such as cloud coverage and type of the clouds, or presence and type of the aerosol layers. Uncertainty of these characteristics could potentially introduce a large error in the determination of the parameters of the primary UHECR particles. Estimate of the typical cloud coverage at the latitude range spanned by the ISS trajectory show that some 70% of all events could be affected by the uncertainty of the atmospheric conditions.

Overview of the JEM-EUSO atmospheric monitor system

To reduce the uncertainty of the atmospheric conditions, JEM-EUSO will be equipped with a dedicated Atmospheric Monitoring (AM) system consisting of (a) an infrared camera and (b) a LIght Detection And Randing (LIDAR) device (see picture). The IR camera and LIDAR data will be complemented by the data of the global atmospheric models calculated on a regular basis for the entire Earth atmosphere by the global meteorological services such as GMAO in U.S. and ECWMF in Europe.

The IR camera will provide an overall view of distribution of clouds in the telescope field of view. It will also measure the brightness temperature of low-altitude optically thick clouds. This will allow determination of the altitude of the clouds based on the known temperature gradient in the troposphere (6°C per km). The infrared camera will be not able to determine optical absorption and scattering properties of the detected clouds. It will be also not able to determine the properties of high altitude clouds, such as cirrus clouds and of low-altitude optically thin structures such as typical aerosol layers. LIDAR measurements will complement the IR camera measurement by detecting and measuring the scattering and absorption properties of the atmospheric layers with optical depth down to 0.15 between the EAS and the JEM-EUSO telescope. At the same time, the LIDAR will provide scattering and absorption profiles of the atmosphere only in several selected directions around the EAS, it will not be able to establish a picture of the distribution of cloud coverage around the EAS. In this respect, the IR camera data will be complementary to the LIDAR data.

The focal surface detector of JEM-EUSO telescope will trigger on the “candidate” EAS events, i.e. transient flashes of UV light appearing anywhere within the 60° field of view of the telescope for some 100 μs and moving across the focal plane with the speed close to 1 pixel per 2.5 μs (linear velocity comparable to the speed of light). Most of the “candidate” events will be not the real UHECR induced EAS, but background events produced by a random coincidence of excess UV emission from the night sky airglow. The rate of the background events will be close to 10 Hz.

The task of JEM-EUSO LIDAR is to probe the optical properties of the atmosphere around the EAS events. To do this, the laser pointing system of the LIDAR will repoint the laser beam in the direction of the last triggered EAS and shoot the laser in several directions around the EAS location. However, it is not known in advance if an arriving trigger is a real EAS event or a background. Thus, the LIDAR will have to repoint to every triggered event within the 60° field of view every 10 s on average. Steering of the laser beam will be done with a tilting mirror.